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Wind Energy Update speaks to Dax Kepshire, vice president and founder of SustainX Energy Storage Solutions, about its compressed air energy storage option for renewable energies.

By Helen Campbell & Rikki Stancich

The idea behind CAES is simple enough: CAES facilities purchase inexpensive off-peak power and use it to compress air into underground salt or limestone caverns. During peak-time, compressed air is drawn from the cavern to drive the turbine.

So why is that, to date, there are only two CAES facilities in operation worldwide (one in Germany and one in the US)? Traditionally, the problem with CAES systems is that they suffer from poor thermal efficiency and that they are geographically constrained, given that they can only be sited where limestone or salt caverns exist.

But SustainX Energy Storage Solutions say it has come up with a way to address the former issue, using an isothermal, rather than abdiatic (i.e. does not exchange heat with its surroundings) approach to gas cycling.

Rather than compressing and expanding the air in turbocompressors and turbines, in which the process is inherently adiabatic, the SustainX system instead compresses and expands the gas within hydraulic cylinders (accumulators and intensifiers), which allows for controlled heat transfer with the ambient surroundings during compression and expansion. Using this approach, SustainX has demonstrated thermal efficiencies greater than 90% for both compression and expansion.

While compressed air energy storage has yet to catch on, its application for solar and wind energy storage suggests that the novel concept may become more mainstream before the decade is out. Wind Energy Update speaks to SustainX energy Solutions’ vice president and co-founder, Dax Kepshire, to learn more.

Wind Energy Update: How does ICAES perform compared to other potential wind energy storage options, such as Xcel's sodium-sulfur battery?

Dax Kepshire: To date, the use of energy storage to support wind in the US has primarily been focused around ramp control and frequency/voltage services. Sodium sulfur batteries can also provide multi-hour wind shifting but their lifetime is severely impacted by the deep depth-of-discharge required for multi-hour energy applications.

SustainX's ICAES offers a superior combination of fast response, capable of addressing ramping issues, and long lifetime even with deep depth-of-discharge.

Wind Energy Update: What is the potential for the ICAES solution to resolve issues surrounding grid integration of renewable energies?

Dax Kepshire: As mentioned above, the ICAES system combines long cycle life at deep depth-of-discharge with a fast response time. Thus, SustainX ICAES can provide ramp control on the seconds and minutes time-scales, and bulk energy shifting on the multi-hour time-scale without shortening system lifetime.

Wind Energy Update: What level of interest have you seen from the wind sector and/or utilities to date, in the ICAES solution?

Dax Kepshire: Specific business development associations, besides the AES/DoE demonstration, are not public at this point, but the ability for SustainX to provide multi-hour and multi-MW storage equivalent to classical CAES without the geological restraints and permitting of caverns/aquifers has drawn considerable interest from developers who are limited by geology or time to installation.

Wind Energy Update: What, in your view, is the potential for CAES to become a real, commercial storage solution for wind energy? Can you quantify the difference it could make?

Dax Kepshire: ICAES is entering the global storage market as an off-the-shelf option within the next two years.

The economics of support for large wind and solar installations will depend on a number of variables, including electricity prices, scale of generation, and quantity and timing of surplus relative to loads.

Wind Energy Update: What is holding it back from greater application?

Dax Kepshire: The market has simply not had access to ICAES previously. Older compressed-air storage technologies have required very large scale, pairing with large natural-gas generators, and locations featuring special geology, such as salt domes in which caverns could be hollowed out.

On these terms, compressed-air energy storage has only proved economic at two locations (one in the US, one in Germany) up to this time.

ICAES liberates compressed air from special geology and fixed scale, since its storage tanks and power units can be deployed almost anywhere and in a modular way that matches power and storage capacity independently to specific needs.

The units can even be relocated as the grid evolves.

Wind Energy Update: Which projects are currently using SustainX CAES technology? Are there any in the pipeline?

Dax Kepshire: SustainX has already demonstrated its technology, and will be preparing a field demonstration of a full-scale, grid-connected ICAES unit at a facility operated by AES Energy Storage, LLC starting in 2012.
 

To respond to this article, please write to the editor: Rikki Stancich